This disclosure relates to circuits for driving light emitting elements, for example, light emitting diodes (LEDs).
LEDs are current-driven devices whose brightness is proportional to their forward current. Forward current can be controlled in various ways. For example, one technique is to use the LED current-voltage (I-V) curve to determine what voltage needs to be applied to the LED to generate a desired forward current. Another technique of regulating LED current is to drive the LED with a constant-current source. The constant-current source can help eliminate changes in current due to variations in forward voltage, which results in constant LED brightness. In this technique, rather than regulating the output voltage, the input power supply regulates the voltage across a current-sense resistor. For example, an operational amplifier can be used to regulate the voltage appearing at the source of a power transistor that is coupled between the current-sense resistor and the LED string. The power supply reference voltage and the value of the current-sense resistor determine the LED current.
One issue that arises in some LED driver circuits is high power consumption. Another issue is that the power transistor typically must be a high-voltage device that is able to withstand the relatively high voltage supply.